The invention relates to a method of positioning a component in a device by means of an actuator, which actuator is provided with two parts which extend in a main plane and are interconnected by means of three bridges, which three bridges can each be shortened in a shortening direction which extends parallel to the main plane in that the bridge is locally heated and subsequently cooled down, in which method the two parts of the actuator are rotated relative to one another about an axis of rotation extending substantially perpendicularly to the main plane through shortening of at least one of said three bridges.
The invention also relates to an actuator suitable for use in such a method, which actuator is provided with two parts which extend in a main plane and are interconnected by means of three bridges, which three bridges can each be shortened in a shortening direction extending parallel to the main plane through local heating of the bridge and subsequent cooling down, while the two parts are rotatable relative to one another about an axis of rotation extending substantially perpendicularly to the main plane through shortening of at least one of said three bridges.
A method and an actuator of the kinds mentioned in the opening paragraphs are known from EP-B-0 638 895. The two parts of the known actuator are plate-shaped and extend in the main plane. The three bridges have the shape of strips and also extend in the main plane. As seen in the main plane, the three bridges are mutually positioned in a kind of timber frame arrangement, a first and a second bridge of the three extending substantially parallel to one another and the third bridge extending obliquely between the two other bridges. According to the known method, the two parts of the actuator are rotated relative to one another in that the first bridge is shortened. This is done through local heating of the first bridge by means of a laser beam and subsequent cooling down. Since the first bridge is shortened, the two parts of the actuator are rotated relative to one another through a very limited angle about an axis of rotation which extends substantially perpendicularly to the main plane and which passes substantially through a point of intersection of the second and the third bridge. Said angle is increased in that this process is repeated a number of times. The two parts of the actuator are rotated in an opposite direction relative to one another about an axis of rotation which passes substantially through a point of intersection of the first and the third bridge in that the second bridge is shortened. Since the actuator has a high rigidity, as seen in the main plane, and a very limited mutual rotation of the two parts of the actuator is obtained as a result of shortening the bridges, a highly accurate positioning is possible by means of the actuator. An incorrectly adjusted angular rotation can be corrected in either direction in that the two parts of the actuator are rotatable relative to one another in two mutually opposed directions. The known method may be used, for example, for accurately adjusting a position of a component, such as a magnetic scanning unit, in a scanning device for magnetic tape information carriers or of a component in an image recording device, such as a CCD image sensor.
A disadvantage of the known method and the known actuator is that a maximum achievable angle of rotation between the two parts of the known actuator is very limited, so that a positioning of components is possible over very limited distances only by means of the known method. This is because a tensile stress arises in a bridge during shortening of this bridge, which stress increases in proportion as the bridge is further shortened. The yield point of the bridge material is locally strongly reduced during heating of the bridge. If the built-up tensile stress in the bridge becomes greater than this reduced yield point, a renewed heating of the bridge will lower the built-up tensile stress owing to plastic deformation of the heated material, i.e. through a lengthening of the bridge. A further buildup of the tensile stress and a further shortening of the bridge are retarded thereby, or even become impossible.
It is an object of the invention to provide a method and an actuator of the kind mentioned in the opening paragraphs whereby again a positioning in two mutually opposed directions is possible with a comparable accuracy, but whereby a considerably greater maximum angle of rotation between the two parts of the actuator is achievable.
To achieve the above object, a method according to the invention is characterized in that the shortening directions of the three bridges are mutually substantially parallel, and the two parts of the actuator are rotated relative to one another by an alternate shortening of two mutually adjoining bridges of the three bridges.
To achieve the above object, an actuator according to the invention is characterized in that the shortening directions of the three bridges are mutually substantially parallel, and the two parts can be rotated relative to one another by an alternate shortening of two mutually adjoining bridges of the three bridges.
Since two mutually adjoining bridges of the three bridges are shortened, the third bridge is deformed, as seen in the main plane, so that the two parts of the actuator are rotated relative to one another about an axis of rotation directed substantially perpendicularly to the main plane and situated adjacent a central portion of the third bridge. The shortening directions of the three bridges of the actuator according to the invention are mutually substantially parallel, with the result that a tensile stress is built up in the two mutually adjoining bridges during shortening of these two bridges. Since the two adjoining bridges are alternately shortened, i.e. are heated in turn, the yield point is lowered in only one of these two bridges each time. A plastic deformation, i.e. a lengthening of the heated bridge under the influence of the tensile stress built up in this bridge, is prevented to a large extent because the non-heated bridge, which has a comparatively high yield point and shows only elastic deformation as a result, temporarily absorbs the tensile stress present in the heated bridge. A shortening of the bridges, once achieved, is thus largely maintained during a renewed heating of one of the bridges, so that the relevant bridge is further shortened during a renewed cooling-down, and the two parts of the actuator can be rotated relative to one another through a comparatively great angle. The object of the invention is furthermore achieved by means of a simple construction and with limited dimensions of the actuator.
A special embodiment of a method according to the invention is characterized in that the two parts of the actuator are rotated relative to one another in a first direction by an alternate shortening of a first and an adjacent second bridge of the three bridges, and in that the two parts of the actuator are rotated relative to one another in a second direction opposed to the first direction by an alternate shortening of the second and the adjacent third bridge. In this special embodiment, said second bridge lies between the first and the third bridge. When the first and the second bridge are alternately shortened, the third bridge is deformed, as seen in the main plane, so that the two parts of the actuator are rotated relative to one another about a first axis of rotation directed substantially perpendicularly to the main plane and situated between the second bridge and the third bridge. Said first direction is defined by the positions of the first and the second bridge with respect to the third bridge. When the second and the third bridge are alternately shortened, the first bridge is deformed as seen in the main plane, so that the two parts of the actuator are rotated relative to one another about a second axis of rotation directed substantially perpendicularly to the main plane and situated between the first bridge and the second bridge. Said second direction is defined by the positions of the second and the third bridge relative to the first bridge and is opposed to said first direction. A relative rotation of the two parts of the actuator in two mutually opposed directions is thus possible in a simple manner by means of a simple and compact construction of the actuator.
A special embodiment of an actuator according to the invention is characterized in that the bridges have a width, seen perpendicularly to the shortening direction, which is substantially smaller than twice a spot diameter of a laser beam used or designed to be used for shortening the bridges. In this special embodiment, a comparatively large portion of the widths of the bridges is heated by the laser beam, so that a comparatively great shortening of the bridges is achieved in a single heating step. This gives the actuator a high effectivity.
A further embodiment of an actuator according to the invention is characterized in that the width of the bridges is at most equal to the spot diameter. In this further embodiment, the bridges are heated by the laser beam in a substantially uniform manner, seen in the width direction, whereby the effectivity of the actuator is further enhanced.
A yet further embodiment of an actuator according to the invention is characterized in that a distance present between the bridges and the width of the bridges have a same order of magnitude. An angle of rotation through which the two parts of the actuator are rotated relative to one another as a result of a shortening of one of the bridges is smaller in proportion as the distance between the bridges is greater, so a comparatively great relative rotation of the two parts of the actuator is obtained upon each shortening of one of the bridges by means of this yet further embodiment of the actuator according to the invention.
A particular embodiment of an actuator according to the invention is characterized in that a distance present between the bridges is substantially greater than the width of the bridges. In this particular embodiment, a comparatively small, but comparatively very accurate relative rotation of the two parts of the actuator is obtained upon each shortening of one of the bridges.
A further embodiment of an actuator according to the invention is characterized in that the actuator is provided with a coupling member which is fastened to one of the two parts of the actuator at a distance from the bridges as seen parallel to the shortening direction and in a fixed position as seen in a displacement direction directed substantially perpendicularly to the shortening direction, which coupling member is uncoupled from the corresponding part when seen in the rotation direction of the actuator. A mutual rotation of the two parts of the actuator is converted into a translation of the coupling member in said displacement direction by means of said coupling member. Since the two parts of the actuator are mutually rotatable in two opposed directions, the coupling member is displaceable also in a direction opposed to said displacement direction. A transmission ratio obtaining between said translation and said rotation is defined by said distance which is present between the coupling member and the bridges, as seen parallel to the shortening direction.
A still further embodiment of an actuator according to the invention is characterized in that the coupling member comprises a blade spring which extends substantially parallel to the displacement direction and substantially perpendicularly to the main plane. Said blade spring has a comparatively low rigidity as seen in a direction parallel to the shortening direction of the bridges. Owing to this low rigidity, a necessary uncoupling in the rotation direction between the coupling member and the two parts of the actuator is provided in a constructionally particularly simple and effective manner.
A particular embodiment of an actuator according to the invention is characterized in that the two parts of the actuator, the three bridges, and the blade spring are manufactured from a single piece of sheeting, the blade spring being bent from the main plane into a position substantially perpendicular to the main plane. The actuator can thus be manufactured in a simple manner as a single integral component.